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I simply love my new 2015 MBA and still wonder why a company as Apple will make such differences in quality experience. Especially the Samsung/LG display thing where the display version in the shops has a Samsung display and the model you bring back home has a LG display.
Nothing nefarious. Apple sells so many devices that they second-source parts. This means they will often have two or more manufacturers making parts, like LG and Samsung for the displays, or Sandisk and Samsung and Toshiba for the SSDs. In case something happens with one supplier, or the factory output isn't high enough, they can still get parts from the second supplier and continue to sell laptops/phones/etc. They can also leverage second sources to get lower prices from the other/primary sources.
The fact that all of the display models at the stores you've seen have Samsung displays probably just means that they all came from the same manufacturing run--logical since they would have had to be delivered to the shops at the same time to set up the displays. I doubt anybody at Apple Headquarters decided that the Samsung displays were better and intentionally shipped only Samsung displays to stores as demo models. I would be surprised if Apple even has such fine-grained control over their inventory like that.
I don't doubt that the LG displays are more "washed out" (i.e., less saturated) than the Samsung displays but I think the other differences you mentioned are probably mostly in your head. With LCD displays, each pixel is controlled individually and I don't see how it's technically possible for the thin borders of Excel cells or thin lines in text to appear fuzzy or blurry. If we were talking about TVs or CRT monitors from 10-15 years ago, then sure. But not LCD panels.
Similarly, I think you only saw a difference in SSD performance because you wanted to. While one SSD might be faster than another, simply booting an app like Excel doesn't read in that much data from the SSD and it isn't read in at a very high rate either--it will be accomplished in a split second no matter which SSD you're using. I doubt the difference between 400 MB/s and 500 MB/s (or whatever) makes a difference of more than a few milliseconds.